The invention relates to an optical multiplexer/demultiplexer, and particularly to an optical multiplexer/demultiplexer which has excellent wavelength flatness in passband, has a wide rejection band, and can function over a wide waveband.
An interleave system, which is one form of advanced wavelength multiplexing communications, requires an optical multiplexer/demultiplexer having a function such that a signal with certain channel wavelength spacings is demultiplexed to two signals with doubled channel wavelength spacings, or conversely, two signals are multiplexed to one signal.
FIG. 11 is an explanatory view showing one example of a prior art technique for coping with this demand. FIG. 11A shows a quartz-based plane optical wave circuit provided on a quartz substrate. This quartz-based plane optical wave circuit comprises four optical couplers 24, 25, 26, 27 and waveguide pairs each comprising two waveguides with different lengths (28, 29), (30, 31), (32, 33), for connecting the optical couplers to each other, that is, a pair of waveguides with different lengths (28, 29) for connecting the optical coupler 24 to the optical coupler 25, a pair of waveguides with different lengths (30, 31) for connecting the optical coupler 25 to the optical coupler 26, and a pair of waveguides with different lengths (32, 33) for connecting the optical coupler 26 to the optical coupler 27. Here due to a difference in optical path length, a phase difference occurs between light, which passes through the waveguide 28, and light which passes through the waveguide 29. The quartz-based plane optical wave circuit is designed so that, when the phase difference caused in the waveguide pair (28, 29) is xcfx86, the phase difference caused in the waveguide pair (30, 31) is 2xcfx86 while the phase difference caused in the waveguide pair (32, 33) is 4xcfx86. There are eight optical paths for allowing light to be input through an input port 20 and to be output through output ports 22, 23. Among them, the shortest optical path is such that light is passed through the waveguide 29, the waveguide 31, and the waveguide 33 in that order. The next shortest optical path is such that light is passed through the waveguide 28, the waveguide 31, and the waveguide 33 in that order. The phase difference in the shortest optical path and the next shortest optical path is xcfx86. Likewise, in the case of third, fourth, fifth, sixth and seventh shortest optical paths and the longest optical path, the phase differences are 3xcfx86, 4xcfx86, 5xcfx86, 6xcfx86, and 7xcfx86, respectively. Here a rectangular, periodic spectral response can be achieved by suitably determining the percentage coupling of optical couplers 24, 25, 26, and 27. Specifically, each term of Fourier series is expressed in terms of a phase difference in each optical path in such a manner that, when a rectangular periodic function is subjected to Fourier series development, the first term is expressed in terms of a component having a phase difference of xcfx86 and the next term is expressed in terms of a component having a phase difference of 2xcfx86. In this case, for the optical couplers, the percentage coupling is determined according to the Fourier coefficient. Thus, when the components for the optical paths are added, a spectral response close to a rectangular shape is provided.
In the case of optical multiplexer/demultiplexers for use in interleave, it is ideal that passband and rejection band are periodically realized as a rectangular spectrum. In short, what is important for the prior art technique is to provide a rectangular spectral response by determining this period through the phase difference xcfx86, taking the phase difference created in the second-stage waveguide pair (30, 31) as 2xcfx86, and taking the phase difference created in the third-stage waveguide pair (32, 33) as 4xcfx86 to properly determine the percentage coupling of the optical couplers.
The conventional multiplexer/demultiplexer is designed so as to function in the best state at a wavelength of 1.545 xcexcm. The optical couplers 24, 25 are directional couplers having a percentage coupling of about 50% at a wavelength of 1.545 xcexcm, the optical coupler 26 is a directional coupler having a percentage coupling of about 98% at a wavelength of 1.545 xcexcm, and the optical coupler 27 is a directional coupler having a percentage coupling of about 2% at a wavelength of 1.545 xcexcm. The waveguide 28 is identical to the waveguide 29 in refractive index and shape of waveguide and is longer by about 2,033 xcexcm than the waveguide 29. Similarly, the waveguide 30 is longer by 4,066 xcexcm than the waveguide 31, and the waveguide 32 is longer by 8,132 xcexcm than the waveguide 33. The waveguides each have a core width of 6 xcexcm and a core height of 6 xcexcm. The difference in specific refractive index between the core and the cladding, xcex94, is 0.8%.
FIGS. 12 and 13 are diagrams showing wavelength loss characteristics for the conventional optical multiplexer/demultiplexer, wherein FIG. 12 shows wavelength loss characteristics for the output port 22 in the case where light is introduced through the input port 20, while FIG. 13 shows wavelength loss characteristics for the output port 23 in the wavelength range of 1.546 xcexcm to 1.550 xcexcm. As can be seen from the drawings, a wavelength-flat passband and a wide rejection band are realized at the designed wavelength around 1.545 xcexcm.
Further, the passband and the rejection band are repeated at periods of about 0.8 nm. This is determined according to the optical path length difference of the waveguides 28, 29.
In the construction wherein the differences in phase between the optical couplers are xcfx86, 2xcfx86, 4xcfx86, however, the use of optical couplers having a high percentage coupling of not less than 50% is unavoidable. That is, this construction should comprise directional couplers having a high percentage coupling. This construction, when applied to practical use, poses the following problems.
FIG. 14 shows spectral characteristics at wavelengths shorter than the designed wavelength, and FIG. 15 spectral characteristics at wavelengths longer than the designed wavelength. As can be seen from the drawings, the level of the rejection band is lowered, and, as compared with the isolation characteristics around the wavelength 1.545 xcexcm, a deterioration in isolation characteristics is significant. The worst isolation value at an ITU-grid wavelength xc2x10.08 nm in the wavelength range of 1.53 xcexcm to 1.56 xcexcm is 17 dB which cannot be said to be satisfactory for practical use. This is attributable mainly to the dependency of the percentage coupling of the optical couplers 24, 25, 26 upon the wavelength. For the optical coupler 27, the percentage coupling is so low that the coupling length is short and the wavelength dependency is small.
FIGS. 16 and 17 are diagrams showing the influence of a dimensional error of the gap between waveguides (Gap in FIG. 11C) in a directional coupler caused in the preparation of the directional coupler, wherein FIG. 16 shows wavelength characteristics in the case where the gap has been narrowed by 0.3 xcexcm, and FIG. 17 wavelength characteristics in the case where the gap has been widened by 0.3 xcexcm. As shown in the drawings, since the isolation characteristics are significantly deteriorated, these directional couplers cannot be used in the optical multiplexer/demultiplexer. This is attributable to the fact that the percentage coupling of the directional coupler is likely to be influenced by the dimensional error. In the case of the optical coupler 27, however, since the coupling length is small, the influence of the error is likely to be relatively small, whereas, for the other optical couplers 24, 25, 26, the isolation characteristics are significantly influenced and deteriorated.
FIGS. 11B and 11C are enlarged views of the directional coupler 26 shown in FIG. 11A, wherein FIG. 11B is an upper plan view and FIG. 11C a cross-sectional view taken on a dotted line of FIG. 11B. In the preparation process of the waveguide, a technique for forming a cladding in the narrow gap between the waveguides (an embedding technique) is very sophisticated. Therefore, embedding of the cladding in the gap between the waveguides is likely to be causative of a deteriorated yield. When Gap in FIG. 11C is larger than the core height of the waveguide, the application of a plasma CVD process becomes possible which can form waveguides having small polarized light dependency. For this reason, the larger the gap between the waveguides, the better the results.
Since, however, in the prior art technique, the percentage coupling of the optical coupler 26 is about 98%, the length of the directional coupler should be not less than 10 mm, for example, when the gap, Gap, between the waveguides is brought to 6 xcexcm for making it possible to apply the plasma CVD process. This leads to an excessively large size of the device which is unrealistic. In the prior art technique, a Gap of about 3.5 xcexcm was adopted to narrow the gap between the waveguides.
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide an optical multiplexer/demultiplexer which has small wavelength dependency, can be used over a wide waveband, is less susceptible to a preparation error, and, at the same time, has good wavelength flatness in passband and a wide rejection band.
In order to attain the above object of the invention, in the optical multiplexer/demultiplexer according to the invention, a construction was adopted wherein the wavelength dependency of the directional coupler and the influence of characteristics sensitive to the preparation error are minimized.
Thus, according to the first feature of the invention, an optical multiplexer/demultiplexer has a construction such that Mach-Zehnders, each comprising four optical couplers each having two input ports and two output ports and disposed in series while connecting the adjacent two optical couplers to each other by two waveguides different from each other in length, are connected in multistage, wherein
on a first connection side, a longer waveguide in the waveguides connecting a first optical coupler to a second optical coupler, a shorter waveguide in the waveguides connecting the second optical coupler to a third optical coupler, and a longer waveguide in the waveguides connecting the third optical coupler to a fourth optical coupler are continuously connected, while on a second connection side opposite to the first connection side, a shorter waveguide in the waveguides connecting the first optical coupler to the second optical coupler, a longer waveguide in the waveguides connecting the second optical coupler to the third optical coupler, and a shorter waveguide in the waveguides connecting the third optical coupler to the fourth optical coupler are continuously connected. This construction can eliminate the need to use directional couplers having a percentage coupling of more than 50%.
Assuming that an MMI (multi mode interference) coupler having a percentage coupling of about 50% is used as the first and second optical couplers while a directional coupler having a percentage coupling of less than 10% is used as the third and fourth optical couplers and, in addition, the difference in phase between two waveguides connecting the first optical coupler to the second optical coupler is xcfx86, the difference in phase between two waveguides connecting the second optical coupler to the third optical coupler and the difference in phase between two waveguides connecting the third optical coupler to the fourth optical coupler are preferably xe2x88x922xcfx86 and 4xcfx86xc2x1xcfx80, respectively.